Compare the osmotic problem and the mechanism of osmotic regulation in freshwater and marine bony fishes.

The osmotic problem in freshwater fishes is the gain of water and loss of salt across thin membranes of the gills because external water has far less salt concentration than internal water. Freshwater fishes (hyperosmotic regulators) have several defenses: 1. Opisthonephric kidney pumps out excess water by forming a very dilute urine. 2. Salt absorbing cells in the gill epithelium move salt ions. This and salt in the food counters diffusive salt loss. Marine fishes (hyposmotic regulators) actually have lower internal salt concentrations than external so they tend to gain salt and lose water. This puts them at risk to dry out. 1. Drink sea water and dispose of the extra salt in two ways: Major sea salt ions (Na, Cl, K) are carried by the blood to the gills where the salts are secreted by salt secretory cells. The remaining ions (Mg, S, Ca) leave the body through feces or kidney excretion.
~A.J.E.